*Includes selected waste factor. Bag counts are rounded up to the nearest whole bag.
function calculateConcrete() {
var len = parseFloat(document.getElementById("slabLength").value);
var wid = parseFloat(document.getElementById("slabWidth").value);
var thick = parseFloat(document.getElementById("slabThickness").value);
var waste = parseFloat(document.getElementById("wasteFactor").value);
var price = parseFloat(document.getElementById("pricePerYard").value);
var resultBox = document.getElementById("result");
var costRow = document.getElementById("costRow");
// Validation
if (isNaN(len) || isNaN(wid) || isNaN(thick) || len <= 0 || wid <= 0 || thick 0) {
var totalCost = totalCubicYards * price;
document.getElementById("resCost").innerHTML = "$" + totalCost.toFixed(2);
costRow.style.display = "flex";
} else {
costRow.style.display = "none";
}
resultBox.style.display = "block";
}
How to Calculate Concrete for Slabs
Whether you are pouring a new patio, a driveway, or a shed foundation, accurate calculation of the concrete volume is critical to project success. Ordering too little results in expensive second deliveries and cold joints, while ordering too much wastes money.
The Concrete Calculation Formula
To determine the amount of concrete needed for a rectangular slab, use the following formula:
Once you have the cubic footage, you typically need to convert this to Cubic Yards, which is how ready-mix trucks measure volume:
Cubic Yards = Cubic Feet / 27
Understanding Thickness
The thickness of your slab depends on its intended use:
4 Inches: Standard for residential patios, sidewalks, and shed bases.
5-6 Inches: Recommended for driveways and areas supporting light vehicles.
6+ Inches: Required for heavy equipment or commercial applications.
Why Include a Waste Factor?
Our calculator allows you to add a "Waste Factor" of 5-10%. This is highly recommended to account for:
Spillage during the pour.
Uneven subgrade (dips in the ground) that require more material to fill.
Some concrete remaining in the truck or mixer.
Pre-Mix Bags vs. Ready-Mix Truck
For small projects (typically under 1 cubic yard), buying pre-mix bags (60lb or 80lb) from a hardware store is often more economical. For projects requiring more than 2 cubic yards, ordering a ready-mix truck is usually more cost-effective and saves significant labor.
Example Calculation: For a 10×10 foot patio at 4 inches thick: